bmp #: vegetated swale - b.f. environmental · pennsylvania stormwater management manual 1 section...

1Pennsylvania Stormwater Management ManualSection 5 - Structural BMPs

Structural BMP Criteria

BMP #: Vegetated Swale

A Vegetated Swale is a broad,shallow, trapezoidal or parabolicchannel, densely planted with avariety of trees, shrubs, and grasses.It is designed to attenuate and insome cases infiltrate runoff volumefrom adjacent impervious surfaces,allowing some pollutants to settle outin the process. In steeper slopesituations, check dams are used tofurther enhance attenuation andinfiltration opportunities.

Pollutant Removalxxxx

Total Suspended Solids:Nutrients:

Metals:Pathogens:

Stormwater FunctionsLow/Med.Low/Med.MediumMed./High

Volume Reduction:Recharge:

Peak Rate Control:Water Quality:

Potential ApplicationsResidential Subdivision:

Commercial:Ultra Urban:

Industrial:Retrofit:

Highway/Road:

YESYESNOLIMITEDYES*YES

Key Design Elements

! Plant dense, low-growing native vegetation that iswater-resistant, drought and salt tolerant, providingsubstantial pollutant removal capabilities

! Longitudinal slopes range from 2 to 6%! Side slopes range from 3:1 to 4:1! Bottom width of 2 to 8 feet (as determined by

Manning's Equation)! Treatment of the 1 inch storm event for water

quality, if designed properly! Check-dams can provide detention storage, as

well as enhanced volume control through infiltra-tion

! Convey the 10-year storm event with a minimum of6 inches of freeboard

! Designed for non-erosive velocities up to the 2-year storm event

! Design to aesthetically fit into the landscape,where possible

*Applicable with specific considerations to design

Other Considerations

! Soil Investigation required

2 Pennsylvania Stormwater Management ManualSection 5 - Structural BMPs

Figure 1. Detail showing vegetated swale

Description

Vegetated swales are broad, shallow, earthen channels designed to slow runoff, promoteinfiltration, and filter pollutants and sediments in the process of conveying runoff. VegetatedSwales provide an environmentally superior alternative to conventional curb and gutterconveyance systems, while providing partially treated (pretreatment) and partially distributedstormwater flows to subsequent BMP�s. Swales are often heavily vegetated with a dense anddiverse selection of native, close-growing, water-resistant plants with high pollutant removalpotential. The various pollutant removal mechanisms of a swale include: filtering by the swalevegetation (both on side slopes and on bottom), filtering through a subsoil matrix, and/orinfiltration into the underlying soils with the full array of infiltration-oriented pollutant removalmechanisms.

A Vegetated Swale typically consists of a band of dense vegetation, underlain by at least 30inches of permeable soil over a 12 to 24 inch aggregate. The permeable soil media should havea minimum infiltration rate of 0.5 inch per hour and contain a high level of organic material toenhance pollutant removal. A non-woven geotextile shall completely wrap the aggregate trench(See Infiltration Trench Section for further design guidelines).


A major concern when designing Vegetated Swales is to make sure that excessive stormwaterflows, slope, and other factors do not combine to produce erosive flows, which exceed VegetatedSwale capabilities. Use of check dams can enhanced swale performance in such situations.

A key feature of vegetated swale design is that they be well integrated into the landscapecharacter of the surrounding area. A vegetated swale can often enhance the aesthetic value of asite through the selection of appropriate native vegetation. Swales may also discreetly blend inwith landscaping features, especially when adjacent to roads.

Variations

Enhanced Vegetated SwaleIn addition to the required elements of a Vegetated Swale, the Enhanced Vegetated Swaleincludes a 12 to 24 inch aggregate bed or trench, wrapped in a non-woven geotextile (SeeInfiltration Trench for further design guidelines). This addition of an aggregate bed or trenchsubstantially increases volume control and water quality performance (both Soil Testing andInfiltration Protocols should be followed) although costs also are increased.

Enhanced Vegetated Swales are best fitted for milder sloped swales where the addition of theaggregated bed system is recommended to make sure that the maximum allowable ponding timeof 48 hours is not exceeded. This aggregated bed system shall consist of at least 12 inches ofuniformly graded aggregate. Ideally, the underdrain system shall be designed like an infiltrationtrench. The subsurface trench should be comprised of terraced levels, though sloping trenchbottoms may also be acceptable. The storage capacity of the infiltration trench may be added tothe surface storage volume to achieve the required storage of the 1-inch storm event.

Grass SwaleGrass swales are essentially conventional drainage ditches. They typically have milder side andlongitudinal slopes than their vegetated counterparts. Grass swales are usually less expensivethan vegetated swales. However, they provide far less infiltration and pollutant removalopportunities. Grass swales are to be used only as pretreatment for other structural BMPs.Design of grass swales is often rate-based, as opposed to volume-based.

Figure 2. Vegetated swale along parking area


Figure 4. Vegetated swale along road (Georgia BMP Manual)

Figure 3. Vegetated swale along residential area (Virginia Stormwater Management Handbook)

Wet SwalesWet swales are essentially linear wetland cells. Their design often incorporates shallow,permanent pools or marshy conditions that can sustain wetland vegetation, which in turn providespotentially high pollutant removal. A high water table or poorly drained soils are a prerequisite forwet swales. The drawback with wet swales, at least in residential or commercial settings, is thatthey may promote mosquito breeding in the shallow standing water. Infiltration is minimal.


Figure 5. Schematic showing vegetated swale along parking area (Cahill conceptual design)

Applications

• • • • • Parking

• • • • • Commercial and light industrial facilities• • • • • Roads and highways• • • • • Residential developments• • • • • Pretreatment for volume-based BMPs• • • • • Alternative to curb/gutter and storm sewer

Design Considerations

1. Vegetated Swales are sized to temporarily store and infiltrate the 1inch storm event, whileproviding conveyance for up to the 10-year storm with freeboard; flows for up to the 2-yearstorm are to be accommodated non-erosively. Swales shall maintain a maximum pondingdepth of 18 inches at the end point of the channel, with a 12-inch average maintainedthroughout. Six inches of freeboard is recommended for the 10-year storm. Residencetimes between 5 and 9 minutes are acceptable for swales without check-dams. Themaximum ponding time is 48 hours, though 24 hours is more desirable (minimum of 30minutes). Swales shall generally be designed for non-erosive velocities up to the 2-yeardesign storm. Likewise, the design treatment depth of flow (for the 1 inch storm) shall notexceed approximately 2/3 the height of swale vegetation. (The design velocity for the 1-inch storm shall be 1 foot per second.) Studies have shown that the maximum amount ofswale filtering occurs for water depths below 6 inches. It is critical that swale vegetationnot be submerged, as it could cause the vegetation to bend over with the flow. This wouldnaturally lead to reduced roughness of the swale, higher flow velocities, and reducedcontact filtering opportunities.

2. Longitudinal slopes between 2% and 3% are generally recommended for swales. If thetopography necessitates steeper slopes, check dams are suggested to reduce the energygradient.


3. Check dams are recommended for vegetated swales with longitudinal slopes greater than3%. They are often employed to enhance infiltration capacity, decrease runoff volume,rate, and velocity, and promote additional filtering and settling of nutrients and otherpollutants. In effect, check-dams create a series of small, temporary pools along thelength of the swale, which shall drain down within a maximum of 48 hours. Swales withcheck-dams are much more effective at mitigating runoff quantity and quality than thosewithout. The frequency and design of check-dams in a swale will depend on the swalelength and slope, as well as the desired amount of storage/treatment volume (1 inchstorm).

Figure 6. Check Dam (Georgia BMP Manual)

Figure 7. Check dams along a vegetated swale (Virginia Stormwater Management Handbook)

Check-dams shall be constructed to a height of 6 to 12 in and be regularly spaced. Thefollowing materials have been employed for check-dams: natural wood, concrete, stone,and earth. Earthen check-dams however, are typically not recommended due to theirpotential to erode. A weep hole(s) may be added to a check-dam to allow the retainedvolume to slowly drain out. Care should be taken to ensure that the weep hole(s) is notsubject to clogging. In the case of a stone check-dam, a better approach might be to allowlow flows (2-year storm) to drain through the stone, while allowing higher flows (10-yearstorm) drain through a weir in the center of the dam. Flows through a stone check-damare a function of stone size, flow depth, flow width, and flow path length through the dam.


The following equation can be used to determine the flow through a stone check dam upto 6 feet long:

q = h1.5 / (L/D + 2.5 + L2)0.5

where:

q = flow rate exiting check damh = flow depthL = length of flowD = average stone diameter in feet (more uniform gradations are preferred)

For low flows, check-dam geometry and swale width are actually more influential on flowthan stone size. The average flow length through a check-dam as a function of flow depthcan be determined by the following equation:

L = (ss) x (2d � h)

where:ss = check dam side slope (maximum 2:1)d = height of damh = flow depth

When swale flows overwhelm the flow-through capacity of a stone check-dam, the top ofthe dam shall act as a standard weir (use standard weir equation). (Though a principalspillway, 6 inches below the height of the dam, may also be required depending on flowconditions.) If the check-dam is designed to be overtopped, appropriate selection ofaggregate will ensure stability during flooding events. In general, one stone size for a damis recommended for ease of construction. However, two or more stone sizes may beused, provided a larger stone (e.g. R-4) is placed on the downstream side, since flows areconcentrated at the exit channel of the weir. Several feet of smaller stone (e.g. AASHTO#57) can then be placed on the upstream side. Smaller stone may also be moreappropriate at the base of the dam for constructability purposes.

4. The effectiveness of a vegetated swale is directly related to the contributing land use, thesize of the drainage area, the soil type, slope, drainage area imperviousness, proposedvegetation, and the swale dimensions. Use of natural low points in the topography may besuited for swale location, as are natural drainage courses although infiltration capabilitymay also be reduced in these situations. The topography of a site shall allow for thedesign of a swale with sufficiently mild slope and flow capacity. Swales are impractical inareas of extreme (very flat or steep) slopes. Of course, adequate space is required forvegetated swales. Swales are ideal as an alternative to curbs and gutters along parkinglots and along small roads in gently sloping terrain.

Siting of vegetated swales should take into account the location and function of other sitefeatures (buffers, undisturbed natural areas, etc.). Siting should also attempt toaesthetically fit the swale into the landscape as much as possible. Sharp bends in swalesshould be avoided.


Implementing vegetated swales is challenging when development density exceeds fourdwelling units per acre, in which case the number of driveway culverts often increases tothe point where swales essentially become broken-pipe systems.

Where possible, construct swales in areas of uncompacted cut. Avoid constructing sideslopes in fill material. Fill slopes can be prone to erosion and/or structural damage byburrowing animals.

5. Soil Investigation and Percolation Testing Required when infiltration is planned and to becredited. (see Section x/x)

6. Guidelines for Infiltration Systems must be met (i.e., depth to water table, setbacks,Loading Rates, etc. (See Section x/x) when infiltration is planned and to be credited.

7. Swales are typically most effective, when treating an area of 1 to 2 acres althoughvegetated swales can be used to treat and convey runoff from an area of 5 to 10 acres insize. Swales serving greater than 10-acre drainage areas will still provide a lesser degreewater quality treatment, unless special provisions are made to manage the increasedflows.

8. The area immediately downstream of the check-dam should be protected from scour witha properly designed rock apron or protective lining. Stabilization on the downstream sideof a check-dam will ensure that the energy associated with high flows is absorbed andthus reduce the potential for resuspension of previously deposited sediments.

9. Runoff can be directed into Vegetated Swales either as concentrated flows or as lateralsheet flow drainage. Both are acceptable provided sufficient stabilization or energydissipation is included (see #6). If flow is to be directed into a swale via curb cuts, providea 2 to 3 inch drop at the interface of pavement and swale. Curb cuts should be at least 12inches wide to prevent clogging and should be spaced appropriately.

10. Vegetated swales are sometimes used as pretreatment devices for other structural BMP�s.However, when swales themselves are intended to effectively treat runoff from highlyimpervious surfaces, pretreatment measures are recommended to enhance swaleperformance. Pretreatment can dramatically extend the functional life of any BMP, as wellas increase its pollutant removal efficiency by settling out some of the heavier sediments.Pretreatment devices generally should be designed for 0.1 inches of runoff per imperviousacre of contributing drainage. This treatment volume is typically obtained by installingcheck dams at pipe inlets and/or driveway crossings. Other pretreatment options include avegetated filter strip, a sediment forebay (or plunge pool) for concentrated flows, or a peagravel diaphragm (or alternative) with a 6-inch drop where parking lot sheet flow is directedinto a swale.

11. The soil base for a vegetated swale must provide stability and adequate support forproposed vegetation. When the existing site soil is deemed unsuitable (clayey, rocky,coarse sands, etc.) to support dense vegetation, replacing with approximately 12 inches ofloamy or sandy soils is recommended. In general, alkaline soils should be used to furtherreduce and retain metals. Swale soils shall also be well-drained. If the infiltration capacityis compromised during construction, the first several feet shall be removed and replacedwith a blend of topsoil and sand to promote infiltration and biological growth.


12. Swales are most efficient when their cross-sections are parabolic or trapezoidal in nature.Swale side slopes are best within a range of 3:1 to 4:1 and shall never be greater than 2:1for ease of maintenance and side inflow from sheet flow.

13. To ensure the filtration capacity and proper performance of swales, the bottom widthstypically range from 2 to 8 feet. Wider channels are feasible only when obstructions suchas berms or walls are employed to prohibit braiding or uncontrolled sub-channel formation.The maximum bottom width to depth ratio for a trapezoidal swale should be 12:1.

14. Published standards suggest that the optimal length of vegetated swales is between 100and 200 ft. There are two acceptable methods of calculating swale length. The firstmethod is based on the design velocity (design flow rate divided by the cross-sectionalarea). In this case, length simply equals the product of the design velocity and the desiredresidence time.

The second method for determining the length of a vegetated swale takes into account therate of infiltration. The following equation only applies for trapezoidal-shaped swales andwas developed for highly permeable soils:

L = (K)(Qavg0.625)(S0.188) / (n0.375)f

where:L = length of swale (ft)Qavg = average flow rate (cfs)S = longitudinal slope (ft/ft)n = Manning�s roughness coefficient (see below)f = infiltration rate (in/hr) � see Infiltration Testing ProtocolK = constant which is a function of side slope parameter, Z (1 vertical / Z horizontal) and isdefined in the following table:

15. Ideal swale vegetation shall consist of a dense and diverse selection of close-growing,water-resistant plants whose growing season preferably corresponds to the wet season.For swales that are not part of a regularly irrigated landscaped area, drought tolerantvegetation should be considered as well. Vegetation shall be selected at an early stage inthe design process, with well-defined pollution control goals in mind. Selected vegetationmust be able to thrive at the specific site and therefore should be chosen carefully.

Seed mixtures that are adapted for a wide range of soil conditions are available.

Z (Side Slope) (1 vertical / Z horizontal)

Swale Length Formula Constant (K)

1 13,650 2 11,900 3 9,900 4 8,500 5 7,500 6 6,750 7 6,150 8 5,680 9 5,255

10 4,955


Use of native plant species is strongly advised, as is avoidance of invasive plant species.Swale vegetation must also be salt tolerant, if winter road maintenance activities areexpected to contribute salt/chlorides.

By landscaping with trees along side slopes, swales can be easily and aestheticallyintegrated into the overall site design without unnecessary loss of usable space. Animportant consideration however, is that tree plantings still allow enough light to pass andsustain a dense ground cover. Though when the trees have reached maturity, they shouldstill provide enough shade to markedly reduce high temperatures in swale runoff.

Table: 1. Commonly used vegetation in swales (New Jersey BMP Manual, 2004)

16. Check the temporary and permanent stability of the swale using the standards outlined inthe Pennsylvania Erosion and Sediment Pollution Control Program Manual. Swales shallconvey either 2.75 cfs/acre or the calculated peak discharge from a 10-year storm event.In most cases, the permissible velocity design method may be used for channel linings.The allowable shear method is also acceptable. Flow capacity, velocity, and design depthin swales are generally calculated by Manning�s equation. Swale lengths between flowobstructions, such as check-dams or driveway crossings, shall still be checked for stability.

Prior to establishment of vegetation, a swale is particularly vulnerable to scour and erosionand therefore its seed bed must be protected with temporary erosion control, such asstraw matting, compost blankets, or fiberglass roving. Most vendors will provideinformation about the Manning�s �n� value and will specify the maximum permissiblevelocity or allowable unit tractive force for the lining material.

The post-vegetation establishment capacity of the swale shall also be confirmed.Permanent turf reinforcement may supersede temporary reinforcement on sites whereattaining the maximum permissible velocity is problematic. If driveways or roads cross aswale, culvert capacity may supersede Manning�s equation for determination of designflow depth. In these cases, the culvert should be checked to establish that the backwaterelevation would not exceed the banks of the swale. If the culverts are to discharge to aminimum tailwater condition, the exit velocity for the culvert should be evaluated for design

Common Name Scientific Name Notes

Alkai saltgrass Puccinellia distans Cool, good for wet, saline swales Fowl bluegrass Poa palustris Cool, good for wet swales Canada bluejoint Calamagrostis canadensis Cool, good for wet swales Creeping bentgrass Agrostis palustris Cool, good for wet swales, salt tolerant Red fescue Festuca rubra Cool, not for wet swales Redtop Agrostis gigantea Cool, good for wet swales Rough bluegrass Poa trivialis Cool, good for wet, shady swales Switchgrass Panicum virgatum Warm, good for wet swales, some salt tolerance Wildrye Elymus virginicus/rigarius Cool, good for shady, wet swales

Notes: These grasses are sod forming and can withstand frequent inundation, and are ideal for the swale or grass channel environment. A few are also salt-tolerant. Cool refers to cool season grasses that grow during the cooler temperatures of spring and fall. Warm refers to warm season grasses that grow most vigorously during the hot, mid-summer months.


conditions. If the maximum permissible velocity is exceeded at the culvert outlet, energydissipation measures must be implemented. The following tables list the maximumpermissible shear stresses (for various channel liners) and velocities (for channels linedwith vegetation). They are taken from the Pennsylvania Erosion and Sediment PollutionControl Program Manual.

Table 2. Maximum Permissible Shear Stresses for Various Channel Liners (PA Erosion and SedimentationControl Program Manual


Figure 8. Manning's n Value with Varying Flow depth (Design of Stormwater Filtering Systems)

17. Manning�s roughness coefficient, or �n� value, varies with type of vegetative cover anddesign flow depth. As a conservative approach, the lower value between that based ondesign depth (see graph) below and that based on vegetative cover (as defined in thePennsylvania Erosion and Sediment Pollution Control Program Manual) shall be used indesign.

Table 3. Maximum Permissible Velocities for Channels Lined with Vegetation (PA E&S Manual)


18. If swales are designed according to the guidelines discussed in this section, high levels ofpollutant reduction can be expected through filtration and infiltration. In a particular swalereach, runoff should be well filtered by the time it flows over a check-dam. Thus, thestabilizing stone apron on the downhill side of the check-dam may be designed as anextension of an infiltration trench. In this way, only filtered runoff will enter a subsurfaceinfiltration trench, thereby reducing the threat of groundwater contamination by metals.

19. Culverts (sometime elevated) are typically used in a vegetated swale at driveway or roadcrossings. By oversizing culverts and their flow capacity, cold weather concerns (e.g.clogging with snow) are minimized. A minimum culvert diameter of 18 inches isrecommended, as is a minimum slope of 1%.

20. Swales shall discharge to another structural BMP (bioretention, infiltration basin,constructed wetlands, etc.), existing stormwater infrastructure, or a stable outfall.

Detailed Stormwater Functions

Infiltration Area (If needed):

Volume Reduction Calculations:The volume retained behind each check-dam can be approximated from the following equation:

Storage Volume = 0.5 x Length of Swale Impoundment Area Per Check Dam x Depth of CheckDam x (Top Width of Check Dam + Bottom Width of Check Dam) / 2

Figure 9. Cross section of vegetated swale


Peak Rate Mitigation:See Section z/z in Section 8 for Peak Rate Mitigation methodology, which addresses link betweenvolume reduction and peak rate control.

Water Quality Improvement:See Section a/a in Section 8 for Water Quality Improvement methodology, which addressespollutant removal effectiveness of this BMP.

Construction Sequence

1. Begin vegetated swale construction only when the upgradient site has been sufficientlystabilized and temporary erosion and sediment control measures are in place. Vegetatedswales should be constructed and stabilized very early in the construction schedule,preferably before mass earthwork and paving increase the rate and volume of runoff.(Erosion and sediment control methods shall adhere to the Pennsylvania Department ofEnvironmental Protection�s Erosion and Sediment Pollution Control Program Manual,March 2000 or latest edition.)

2. Rough grade the vegetated swale. Only the lightest, least disruptive equipment may beused, to avoid excessive compaction and/or land disturbance. Excavating equipmentshould operate from the side of the swale and never on the bottom. If excavation leads tosubstantial compaction of the subgrade (where an infiltration trench is not proposed), thefirst several feet shall be removed and replaced with a blend of topsoil and sand topromote infiltration and biological growth. At the very least, topsoil shall be thoroughlydeep plowed into the subgrade in order to penetrate the compacted zone and promoteaeration and the formation of macropores. Following this, the area should be disked priorto final grading of topsoil.

3. Construct check dams, if required.

4. Fine grade the vegetated swale. Accurate grading is crucial for swales. Even the smallestnon-conformities may compromise flow conditions.

5. Seed and vegetate according to final planting list. Plant the swale at a time of the yearwhen successful establishment without irrigation is most likely. However, temporaryirrigation may be needed in periods of little rain or drought. Vegetation should beestablished as soon as possible to prevent erosion and scour.

6. Concurrent with #7, stabilize freshly seeded swales with appropriate temporary orpermanent soil stabilization methods, such as erosion control matting or blankets. Erosioncontrol for seeded swales shall be required for at least the first 75 days following the firststorm event of the season. If runoff velocities are high, consider sodding the swale ordiverting runoff until vegetation is fully established. (Erosion and sediment controlmethods shall adhere to the Pennsylvania Department of Environmental Protection�sErosion and Sediment Pollution Control Program Manual, March 2000 or latest edition.)

7. Once the swale is sufficiently stabilized, remove temporary erosion and sediment controls.It is very important that the swale be stabilized before receiving upland stormwater flow.

8. Follow maintenance guidelines, as discussed below.


Note: If a vegetated swale is used for runoff conveyance during construction, it must be regradedand reseeded immediately after construction and stabilization has occurred. Any damaged areasmust be fully restored to ensure future functionality of the swale.

Maintenance Issues

Compared to other stormwater management measures, the required upkeep of vegetated swales isrelatively low. In general, maintenance strategies for swales focus on sustaining both the hydraulicand pollutant removal efficiency of the channel, as well as maintaining a dense vegetative cover.Experience has proven that proper maintenance activities ensure the functionality of vegetatedswales for many years. The following schedule of inspection and maintenance activities isrecommended:

Activity Schedule

- Inspect and correct erosion problems, damage to vegetation, and sediment and debris accumulation (when > 3 in at any spot or covers vegetation) - Inspect vegetation on side slopes for erosion and formation of rills or gullies, correct as needed - Inspect for pools of standing water; dewater and discharge to a sanitary sewer at an approved location - Mow and trim vegetation to ensure safety, aesthetics, proper swale operation, or to suppress weeds and invasive vegetation; dispose of cuttings in a local composting facility; mow only when swale is dry to avoid rutting - Inspect for litter; remove prior to mowing - Inspect for uniformity in cross-section and longitudinal slope, correct as needed - Inspect swale inlet (curb cuts, pipes, etc.) and outlet for signs of erosion or blockage, correct as needed

Annually or 48 hours after every major storm event (Semi-annually the first year)

- Plant alternative grass species in the event of unsuccessful establishment - Re-seed bare areas; install appropriate erosion control measures when native soil is exposed or erosion channels are forming - Roto-till and replant swale if draw down time is less than 48 hours - Inspect and correct check dams when signs of altered water flow (channelization, obstructions, etc.) is identified - Water during dry periods, fertilize, and apply pesticide only when absolutely necessary

As needed


Most of the above maintenance activities are reasonably within the competence of individualhomeowners. More intensive swales (i.e. more substantial vegetation, check dams, etc.) may naturallywarrant more intensive maintenance duties and should be vested with a responsible agency. Alegally binding and enforceable maintenance agreement between the facility owner and the localreview authority might be warranted to ensure sustained maintenance execution. Winter conditionsalso necessitate additional maintenance concerns, which include the following:

• Inspect swale immediately after the spring melt, remove residuals (e.g. sand) and replacedamaged vegetation without disturbing remaining vegetation.• If roadside or parking lot runoff is directed to the swale, mulching and/or soil aeration/manipulation may be required in the spring to restore soil structure and moisture capacityand to reduce the impacts of deicing agents.• Use non-toxic, organic deicing agents, applied either as blended, magnesium chloride-based liquid products or as pretreated salt.• Use salt-tolerant vegetation in swales.

Cost Issues

As with all other BMPs, the cost of installing and maintaining Vegetated Swales varies widely withdesign variability, local labor/material rates, real estate value, and contingencies. In general,Vegetated Swales are considered relatively low cost control measures. Moreover, experience hasshown that Vegetated Swales provide a cost-effective alternative to traditional curbs and gutters,including associated underground storm sewers. The following table compares the cost of atypical vegetated swale (15 ft top width) with the cost of traditional conveyance elements.

Source: Bay Area Stormwater Management Agencies Association (June 1997)

It is important to note that the costs listed above are strictly estimates and shall be used for designpurposes only. Also, these costs do not include the cost of activities such as clearing, grubbing,leveling, filling, and sodding (if required). The Southeastern Wisconsin Regional PlanningCommission (SEWRPC, 1991) reported that actual costs, which do include these activities, mayrange from $8.50 to $50.00 per linear foot depending on swale depth and bottom width. When allpertinent construction activities are considered, it is still likely that the cost of vegetated swaleinstallation is less than that of traditional conveyance elements. When annual operation andmaintenance costs are considered however, swales may prove the more expensive option,though they typically have a much longer lifespan.

Swale Underground Pipe Curb & Gutter

Construction Cost (per linear foot)

$4.50 - $8.50 (from seed) $15 � 20 (from sod)

$2 per foot per inch of diameter (e.g. a 12� pipe would cost $24 per linear foot)

$13 � 15

Annual O&M cost (per linear foot)

$0.75 No data No data

Total annual cost (per linear foot)

$1 (from seed) $2 (from sod)

No data No data

Lifetime (years) 50 20


Specifications

The following specifications are provided for information purposes only. These specificationsinclude information on acceptable materials for typical applications, but are by no meansexclusive or limiting. The designer is responsible for developing detailed specifications forindividual design projects in accordance with the project conditions.

1. Swale Soil shall be USCS class ML (Inorganic silts and very fine sands, rock flour, silty orclayey fine sands with slight plasticity), SM (Silty sands, poorly graded sand-silt mixtures),SW (Well-graded sands, gravelly sands, little or no fines) or SC (Clayey sands, poorlygraded sand-clay mixtures). The first three of these designations are preferred for swalesin cold climates. In general, soil with a higher percent organic content is preferred.

2. Swale Sand shall be ASTM C-33 fine aggregate concrete sand (0.02 in to 0.04 in).

3. Check dams constructed of natural wood shall be 6 in to 12 in in diameter and notched asnecessary. The following species are acceptable: Black Locust, Red Mulberry, Cedars,Catalpa, White Oak, Chestnut Oak, Black Walnut. The following species are notacceptable, as they can rot over time: Ash, Beech, Birch, Elm, Hackberry, hemlock,Hickories, Maples, Red and Black Oak, Pines, Poplar, Spruce, Sweetgum, and Willow. Anearthen check dam shall be constructed of sand, gravel, and sandy loam to encouragegrass cover (Sand: ASTM C-33 fine aggregate concrete sand 0.02 in to 0.04 in, Gravel:AASHTO M-43 0.5 in to 1.0 in). A stone check dam shall be constructed of R-4 rip rap, orequivalent.

4. Develop a native planting mix. (see Appendix XX)

5. Topsoil amended with compost (see Soil Restoration Section)

6. If infiltration trench is proposed, see Infiltration Trench Section for specifications.

ReferencesAlameda Countywide Clean Water Program (ACCWP). �Grassy Swales.� Catalog of

Control Measures. <http://www.oaklandpw.com/creeks/pdf/Grassy_Swales.pdf>

AMEC Earth and Environmental Center for Watershed Protection et al. GeorgiaStormwater Management Manual. 2001.

California Stormwater Quality Association. California Stormwater Best ManagementPractices Handbook: New Development and Redevelopment. 2003.

Caraco and Claytor. Stormwater BMP Design Supplement for Cold Climates. 1997.

City of Portland Environmental Services. City of Portland Stormwater Management Manual:Revision #2. 2002.

Center for Watershed Protection and Maryland Department of the Environment. 2000Maryland Stormwater Design Manual. Baltimore, MD: 2000.


Claytor, R.A. and T.R. Schuler. Design of Stormwater Filtering Systems. Center forWatershed Protection. Silver Spring, MD: 1996.

Colwell, S. R. et al. Characterization of Performance Predictors and Evaluation of MowingPractices in Biofiltration Swales. 2000.

DURMM Manual 2002

Fletcher, T., Wong, T., and Breen, P. �Chapter 9 � Buffer Strips, Vegetated Swales andBioretention Systems.� Australian Runoff Quality (Draft). University of New Castle �Australia.

Lichten, K. �Grassy Swales.� BMP Fact Sheets. Bay Area Stormwater Management AgenciesAssociation (BASMAA). 1997.

Maine Department of Transportation. Maine Department of Transportation BMP Manual forErosion and Sedimentation Control. 1992.

North Central Texas Council of Governments. Stormwater Best Management Practices: AMenu of Management Plan Options for Small MS4s in North Central Texas. 2002.

Schueler, T. et al. A Current Assessment of Urban Best Management Practices: Techniquesfor Reducing Nonpoint Source Pollution in the Coastal Zone. 1992.

United States Environmental Protection Agency (USEPA). �Post-Construction Storm WaterManagement in New Development & Redevelopment.� National Pollutant DischargeElimination System (NPDES). <http://cfpub1.epa.gov/npdes/stormwater/menuofbmps/post_8.cfm>

United States Environmental Protection Agency (USEPA). Storm Water Technology FactSheet: Vegetated Swales (EPA 832-F-99-006). 1999.

Vermont Agency of Natural Resources. The Vermont Stormwater Management Manual.2002.

Virginia Stormwater Management Handbook, Volumes 1 and 2, First Edition, 1999

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